The present invention relates to transparent touch panels.
Touch panels are well known. These panels generally comprise two flexible membrane elements, each element either being electrically conductive or being fabricated from a flexible plastic film coated with a thin layer of conductive material. The panels can be flat, or curved such as being formed to fit over a curved surface of a display.
For applications requiring a transparent touch panel, the plastic film can be polyethylene terephthalate film and the conductive material can be a metal such as gold, or a metal oxide such indium tin oxide. The membrane elements are oriented such that the conductive coatings face each other. By selective patterning processes, the two conductive coating surfaces can together form an addressable analog array or a matrix switch array. Operation of the touch panel is initiated by a user forcing the conductive surfaces of the elements into point contact. A matrix type touch panel has a fixed number of selectable discrete row-column coordinates, and the touch point is detected digitally. In the analog version, each element is alternately driven from a power source through a network of opposed busbars, whereby signals are generated which represent the X and Y positions of the touch point. The analog type touch panel has substantially continuous selectable coordinates.
Transparent touch panels can be used for computer data input in combination with a computer display. The touch panel is placed in front of the display. When a program menu is shown on the display, the user can input the appropriate choice by pressing the panel in the area over the correct choice. In this type of application, an advantage of a touch panel is that a large number of control knobs and buttons and switches can be replaced by a single touch panel.
A problem in using displays in outdoor or other high ambient light applications is the contrast problem created by the reflections of the high ambient light competing with the light intensity of the computer display. For this reason, light emitting displays, such as CRTs, plasma displays, or electro-luminescent displays, etc., are usually used in high ambient light applications. However, the contrast problem can still be unacceptable when these displays are used, such as in automobiles where the display can be in bright sunlight.
Contrast ratio is one measure of the readability of a display. It is defined as the ratio of the brightness of the displayed image to the brightness of the background. The human eye can adjust to different light intensities within a reasonable range. Therefore a display can be dim and can still be readable, as long as there is sufficient contrast between the image displayed and the background. The contrast ratio can be improved by either increasing the brightness of the displayed image or by reducing the brightness of the background, or both. A major contributor to the brightness of the background is reflected ambient light. A reduction in the reflection of ambient light will generally improve the contrast ratio and thus readability.
The use of a "grating" of limited thickness such as mini louvres or a mesh, such as the black microweave screen taught in U.S. Pat. No. 4,253,737 to Thomsen et al., placed in front of the display, is quite effective in preventing incident ambient light from causing unwanted reflections. These materials work by virtue of the favorable relationship of the angles of the unwanted ambient light (relatively acute) vs. that of the display-to-user's eye light path (relatively normal to the plane of the display). A large amount of the ambient light is absorbed by the grating while the light emitted by the display is relatively unaffected.
However, the use of dark colored microweave screens is usually precluded when transparent touch panels are used in front of displays, because these microweave screens are susceptible to contamination from dirt, spilled liquids, finger oil, etc., which are inherent in touch panel operation. A dirty microweave screen detracts from the information displayed, and can be objectionable aesthetically. Because of its fine mesh, a dirty microweave screen can be very hard to clean.
In view of these problems, there is a need for a contrast enhancing transparent touch panel that can be used in applications having a high level of ambient light.